Examples of currently commercialized secondary cells include nickel-cadmium cells, nickel-metal hydride cells, nickel-zinc cells, and lithium secondary cells. Among such various secondary cells, lithium secondary cells are freely rechargeable because of having substantially no memory effect compared with nickel-based secondary cells, and have a very low self-discharge rate and high energy density. Owing to these merits, there has been high interest in lithium secondary cells.
In general, lithium secondary cells use a lithium-based oxide as a positive electrode active material and a carbonaceous material as a negative electrode active material. A lithium secondary cell may include: an electrode assembly in which a positive electrode plate coated with such a positive electrode active material and a negative electrode plate coated with such a negative electrode active material are disposed with a separator therebetween; and a case, that is, a cell case in which the electrode assembly and an electrolytic solution are sealed.
In general, according to case types, lithium secondary cells may be classified into a can type in which an electrode assembly is accommodated in a metal can, and a pouch type in which an electrode assembly is accommodated in a pouch formed of an aluminum laminate sheet.
In recent years, secondary cells have been widely used not only in small-sized devices such as portable electronic devices, but also in medium to large-sized devices such as automobiles or power storage devices. For use in such medium to large-sized devices, a large number of secondary cells may be electrically connected to increase capacity and output power. In particular, pouch-type secondary cells are widely used in medium to large-sized devices owing to merits such as lightness and ease of stacking.
In the related art, when a battery pack is constructed by stacking a plurality of pouch-type secondary cells to form a cell assembly and accommodating the cell assembly in a pack case, end plates may be provided on both the outermost sides of the cell assembly in the stacking direction of the pouch-type secondary cells. In general, such end plates are formed of a metallic material and have a function of protecting and fixing secondary cells and cartridges and maintaining surface pressure.
However, in such a battery pack structure of the related art, it is necessary to fix a cell assembly and a pack case to each other in addition to fixing end plates to the cell assembly. Typically, the cell assembly may be fixed by using fastening members such as bolts and nuts between the pack case and the end plates of the cell assembly. In this case, additional spaces are necessary in the pack case and the end plates for such fastening members. In addition, it may be necessary to form or prepare an additional space in the pack case for a process of fastening the cell assembly to the pack case. However, since these spaces are not used to accommodate secondary cells, the energy density of the battery pack may be decreased, and the structure of the battery pack may be complicated.
In addition, the structure in which the end plates and the pack case are fixed using fastening members may not be effective in stably maintaining a fixed state of the cell assembly in the pack case because the fastening members may become loose as the battery pack is exposed to vibrations or impacts during use. This may cause breakage or damage of various components included in the battery pack, and thus the battery pack may malfunction or break down.
Furthermore, the battery pack structure of the related art makes it difficult to assemble the end plates and the pack case and requires the use of additional fastening members for coupling the end plates and the pack case, and a process of previously placing such fastening members on the pack case is not easy to perform, thereby decreasing productivity and increasing manufacturing costs.
Furthermore, in general, pack cases of battery packs accommodating cell assemblies are formed of a plastic material, and are thus vulnerable to impacts or the like because of low rigidity.